The invention relates to a high-pressure sodium lamp provided with a ceramic discharge vessel which is enclosed with intervening space by an outer bulb, which space is provided with a gas filling, a pair of electrodes being arranged in the discharge vessel each connected to a current supply conductor issuing from the discharge vessel at a respective end, between which electrodes a central portion of the discharge vessel extends with a volume V and a length L, the discharge vessel being provided with a filling of an amalgam with a quantity m.sub.Na of sodium by weight and a quantity m.sub.Hg of mercury by weight.
Such a lamp is known from Neues aus der Technik, No. 4, 15 December 1989. By "ceramic discharge vessel" is meant a discharge vessel of a refractory material such as monocrystalline metal oxide, for example sapphire, polycrystalline metal oxide, for example translucent gastight aluminium oxide (PCA) or yttrium oxide, and crystalline non-oxidic material such as aluminum nitride. The gas filling in the space between the discharge vessel and the outer bulb contributes to the heat transport from the discharge vessel to the surroundings. This has the advantage that a small discharge vessel can be used, so that the light radiated by the lamp can be very well concentrated into a beam and a high system efficiency can be achieved. A gas filling, however, increases the influence of the ambient temperature on the temperature of the discharge vessel, and thus leads to greater variations in the arc voltage of the lamp in the case of temperature fluctuations in the surroundings. It is stated for the unsaturated lamp, in which the sodium and the mercury are fully evaporated during evaporation, that the vapor pressure and thus the arc voltage and other lamp characteristics remain substantially constant in the case of temperature fluctuations. The use of a gas filling is for this reason recommended for an unsaturated lamp in the above publication.
For outdoor use, it is required for the unsaturated lamp that the amalgam be fully evaporated during operation at comparatively low ambient temperatures. Because of the small quantity of amalgam, the unsaturated lamp requires high temperatures at the ends of the discharge vessel which to realize vapor pressures which correspond to those of a comparable saturated lamp. These high temperatures in unsaturated lamps with PCA discharge vessels have an additional advantage because they achieve a comparatively high aluminum vapor pressure. The comparatively high aluminum vapor pressure decelerates parasitic chemical reactions between wall material and sodium, by which it is prevented that the quantity of sodium, which is already small, is lost in this way. Metal lamp components such as lead-through elements, soldered connections between the electrodes and the lead-through elements, and portions of current supply conductors adjoining the ends of the discharge vessel are also strongly heated at the prevailing high temperatures. Although nitrogen is regarded as an inert gas in the cited publication, it was found that this gas attacks the said metal lamp components under the prevailing conditions in the known lamp, which leads on the one hand to brittleness and on the other hand to a volume increase of the attacked component. The volume increase of lead-through elements involves the risk of the discharge vessel cracking and starting to leak. The embrittlement of a soldered joint will reduce the strength thereof and may even lead to a connection being completely broken. In the case of an electrode fastened by means of a soldered joint, this leads to tilting of the electrode, which means the end of lamp life. A disadvantage of argon and other rare gases is that breakdown occurs therein under certain circumstances when an ignition voltage for starting the lamp is applied, which renders ignition difficult and adversely affects lamp life.